<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .warning {
            border: 1px solid red;
        }

        div.warning {
            border: 2px solid blue;
        }
    </style>
</head>

<body>
    <div class="warning">This is a warning.</div>
</body>

</html>